Beaver Island shoreline in northern Lake Michigan. Photo by Lisa Borre.
News reports about warming lake temperatures began to trickle into my world lakes news feed as the summer heated up this year. I read stories about warmer than normal lakes in North America and Europe, including lakes in Kansas, California, and Washington. By the end of July, the Large Lakes Observatory at the University of Minnesota Duluth reported that Lake Superior’s average surface temperature was 8-10°F above average and expected to stay above normal through the remainder of summer.
I had recently returned from a visit with family on Beaver Island in northern Lake Michigan. This was not the first time in recent years that I found the lake surprisingly comfortable for swimming in early July. I was tempted to write it off as an unusually warm summer, which it was. But the news reports comparing lake temperatures to historical records and my own experience at a beach I’ve visited for over 40 years made me wonder what the latest research tells us about the effects of global climate change on lakes. Are lakes warming, and if so, what are the consequences?
Global assessment shows 95% of lakes are warming
In 2010, National Geographic News reported on the results of the first comprehensive global study of lake temperature trends. The study — conducted by researchers at NASA’s Jet Propulsion Laboratory (JPL) in California using satellite data — found that in the last 25 years, the world’s largest lakes have been steadily warming, some by as much as 4°F (2.2°C). In some cases, the trend is twice as fast as the air temperature trend over the same period.
Simon J. Hook, one of the scientists who led the JPL study of 150 lakes, became interested in comparing his remote sensing data with actual in-lake measurements, especially on some of the more rapidly changing lakes, to determine what types of ecological changes were occurring. He connected with researchers working on lakes as climate indicators through the Global Lake Ecological Observatory Network (GLEON). Established in 2005, GLEON scientists are creating a network of instrumented buoys to monitor lakes worldwide.
Recognizing the need for a collaborative approach to researching changes in lake temperature and to studying the ecological effects of climate change, they formed the Global Lake Temperature Collaboration (GLTC) in the fall of 2010. The GLTC has since grown to over 50 investigators studying lakes all over the world, including the Great Lakes and Lake Tahoe in North America, Lake Baikal in Russia, and Lake Tanganyika in East Africa. The group held their first meeting in June at the University of Nebraska-Lincoln (UNL) and began pooling data and expertise to better understand global changes in lake temperature.
“Recent studies have revealed significant warming of the world’s lakes,” said workshop organizer John Lenters, a climatologist and lake scientist at UNL and member of GLEON. “The potential impacts of these changes on lake ecosystems make it increasingly important for scientists with access to global lake temperature records to assemble and synthesize relevant data,” he said in a statement before the meeting.
Detailed results of the workshop will be published in the coming weeks and months, but organizers were pleased by the enthusiasm among the workshop participants from 11 countries. They also discovered new data sets for lakes in Europe and elsewhere. “Lake Peipsi, on the border between Estonia and Russia, was one of the lakes in our global study,” said Hook. “It also has a long-term in situ record that we didn’t know existed.” Researchers from Estonia attended the workshop and are now sharing their data as part of the collaborative effort to learn more about warming lake temperatures.
“The number of lakes with both satellite and in situ measurements more than doubled as a result of the workshop,” said Hook. The in-lake measurements support the findings of the remote sensing data: 95% of lakes around the world are warming.
Lakes in North America and northern Europe warming more rapidly
A second global assessment, which used different remote sensing techniques than the JPL study, was published in 2011. The ARCLAKE study used European satellite data over a shorter time period, from 1992 to 2011.
In a special supplement to the Bulletin of the American Meteorological Society published in July 2012, the annual “State of the Climate” report compared the results of the two studies. “Despite differing periods and methods, both ARCLAKE and [the JPL study] are consistent in identifying relatively rapid warming in lakes of both North America and Europe,” the report concluded.
“It’s good that we have two global studies using different methods and data to verify the results,” said Hook. The ARCLAKE researchers are also involved in the GLTC.
Lake water temperatures warming more rapidly than air temperatures
The data from in-lake measurements confirm that water temperatures are warming more rapidly than air temperatures for many lakes around the world.
The fact that lake water is warming more rapidly than air seemed counterintuitive to this lake swimmer. Based on first-hand experience, I know that the warming of lake water in northern climates always lags behind the air in the spring. And in the fall, the lake remains warmer long after the cool autumn breezes begin to blow. When thinking about climate change versus the annual weather cycle, I expected that lake temperatures would warm more slowly than the air. I was surprised to learn that the opposite is true.
Hook explained that lakes integrate changes over time and lake temperatures change more gradually whereas air temperatures change very rapidly. “Lakes are great integrators,” he said. “Measuring lake temperature takes into account all of the variables, such as air temperature, amount of snow melt, the effects of surrounding forest fires, and the amount of biological activity in the water.”
Researchers are also looking at whether changes in other climatic variables may help explain the more rapid warming of lake water.
Lakes are good indicators of climate trends
Just as sailors use their barometers to help predict changes in weather and to understand sometimes fickle winds, climate scientists are finding lakes to be very good indicators of climate change. In addition to being able to integrate changes among many different environmental variables at once, researchers can study these changes relatively easily with the aid of modern technologies. “We can measure lake temperatures from space very accurately and see warming and cooling trends without the rapid daily changes seen with air temperatures,” Hook told me.
Although researchers need 20 years of data to understand climate trends, a growing network of in-lake buoys monitor lakes in near real-time. The buoy data from places like Lake Tahoe, the Great Lakes and GLEON sites can be compared to the satellite data to gain a historical perspective, and eventually will serve as the basis for a long-term record for the lakes themselves.
Our understanding of how lake temperatures are changing around the world has greatly improved in the past few years and will continue to improve through the various collaborative efforts now underway.
The consequences of warming lakes
Warming lakes are already experiencing water quality problems and increases in toxic algae blooms. Experts predict that the loss of ice cover on some lakes is expected to increase, impacting water quality and possibly lake levels. The potential consequences are enormous when considering how many lakes are affected.
The results of the global assessment of lake temperatures helped researchers working on lakes put their own findings into context. Lake experts are now trying to share their experiences to create a better understanding of the effects of climate change on lakes around the world.
When Simon Hook began studying lakes, he was looking for a better way to understand the effects of climate change. After the results of the JPL study were released, he was pleasantly surprised to find out how interested people were. “People value lakes highly and are concerned about what’s happening to their lake,” he told me. Perhaps this explains some of the enthusiasm for forums such as the GLTC and GLEON. And maybe it will help the groups studying lakes to secure much-needed funding to maintain the satellites, network of buoys and long-term monitoring programs, and to support the unprecedented global collaboration currently underway.
I now realize that it’s too simplistic to look at warming trends in lakes from the standpoint of water temperature alone. The potential ecological consequences are far greater and more complex than just a few degrees of temperature change might imply. In future posts, I’ll explore further how climate change is affecting lakes around the world.
For more information about warming lake temperatures, visit www.laketemperature.org.